Method for manufacturing high-strength steel sheet parts subject in use to fatigue stresses

ABSTRACT

The manufacturing method comprises the steps of: carrying out one or more forming operations so as to give the desired geometry to the part; and subjecting the part thus formed to a single heat treatment having only a stress relieving treatment, which is preferably carried out at a temperature in the range from 530° C. to 580° C. for a time in the interval from 45 to 60 minutes and is followed by cooling of the part in air. By virtue of the formed part being subjected to a stress relieving heat treatment, the residual stress state due to the initial forming process and to the bead welding, if any, is eliminated or at least significantly reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a “national phase” application based uponInternational Patent Application PCT/EP2012/052757 filed on Feb. 17,2012, which, in turn, claims priority to Italian Patent ApplicationTO2011A 000139 filed on Feb. 18, 2011.

BACKGROUND OF INVENTION 1. Field of Invention

The present invention relates to a method for manufacturinghigh-strength steel sheet parts suitable for fatigue applications, i.e.parts which are subject in use to fatigue stresses. In particular,although not exclusively, the invention relates to the manufacturing ofhigh-strength steel sheet parts intended to be used on vehiclesuspension systems. In the following description and claims, theexpression “high-strength” is to be intended as referred to a steelhaving a yield strength greater than 420 MPa.

2. Description of Related Art

As is known, plastic forming operations carried out on steel sheetsleave stress states in the material, the so-called residual stresses,which turn out to be of the same order of magnitude as the yieldstrength of the material. The same occurs when a weld bead is made on asteel sheet: depending on how much the welded joint is free to deformafter cooling, residual stresses are generated in the material which aregreat in the vicinity of the weld bead and decrease as the distance fromthe weld bead increases. These residual stress states of the materialremain in the finished part throughout its service life. If the finishedpart is requested to bear fatigue stresses (as is the case, forinstance, with vehicle suspension parts), then the residual stress stateusually reduces the ability of the part to meet the prescribedrequirements in terms of fatigue life. The higher the quality of thesteel of which the part is made, the larger the effect of reduction inthe fatigue life of the part. This is due to the fact that the fatiguelimit is less dependent on the yield strength of the material than theresidual stress level, which on the contrary is directly linked to theyield strength of the material. In order to compensate the reduction inthe fatigue life due to the residual stress state in the material,either the thickness of the material is increased (with resulting impacton the weight and cost of the part) or special materials are adopted(for instance, boron steel) which are subjected to heat treatment,namely to quenching followed by tempering, such a heat treatmentallowing to obtain high mechanical properties and at the same time toreduce (or even eliminate) the residual stress state due to the formingprocess and to the quenching treatment.

In case of metal sheet parts for vehicle suspension systems obtained byforming tubular blanks, the manufacturing method comprises first of alla so-called tube making phase, i.e. a phase of production of a tubularblank, which typically consists in the following operations:

-   -   1) producing ribbons having a width equal to the required        circumference of the tube starting from a coil of cold- or        hot-rolled steel sheet;    -   2) unwinding the ribbons and producing the tube continuously by        carrying out in sequence forming operations;    -   3) welding the tube longitudinally (i.e. along a direction        parallel to the axis of the tube) without addition of material,        and also scarfing the tube, if necessary; and    -   4) straightening the tube.

The tube blank thus obtained is then formed (typically by cold-stampingand/or hydro-forming) so as to be given the desired final geometry. Forinstance, in case of a cross-member for a twist-beam axle rearsuspension for a motor-vehicle, the tube blank is bent and/or squashedso that its own longitudinal axis is given the desired course and/or itscross-section is given the desired shape. In case of use of normalstructural steels (for instance Fe510D) or of steels for heat treatment(for instance 20MnB5), a normalizing treatment is performed beforeforming in order to reduce the high residual stress state generated bythe tube making process, which is typically slightly less than the yieldstrength of the material. Moreover, in case of steels for heat treatmentthe formed part is finally subjected to a quenching and temperingtreatment, in order to reach the high mechanical properties required bythe specific mission of the part. A shot-peening operation, ifnecessary, may follow the quenching and tempering treatment in order tofurther increase the fatigue life of the part. In case of use of normalstructural steels, the parts thus obtained have low mechanicalproperties, but low production costs, whereas the use of steels for heattreatment allows to obtain parts having very high mechanical properties,but high production costs.

A method for manufacturing a steel tube is known from EP 2 045 348,wherein the tube is first subjected to hot-bending (at a temperature ofabout 1000° C.) and finally to a quenching and tempering treatment. Asimilar method is known from JP 7 090375, since according to thisdocument the steel tube is also first hot-formed and then subjected to aquenching and tempering treatment. U.S. Pat. No. 3,533,157 discloses amethod for manufacturing tubes and mentions, as the only heat treatmentused in the method, the normalizing treatment, which involves, as isknown, heating up to a temperature higher (typically by about 50° C.)than the critical zone or critical interval of the steel.

SUMMARY OF INVENTION

It is the object of the present invention to provide a method formanufacturing steel sheet parts subject in use to fatigue stresses, suchas for instance parts for vehicle suspension systems, which allows toensure that high mechanical properties are obtained, which are muchhigher than those obtainable using normal structural steels, but whichrequires less time and lower costs than those required in case of use ofsteels for heat treatment.

To this end, the method of the present invention includes the steps ofcarrying out one or more forming operations on a sheet of high-strengthsteel so as to provide the part with the desired geometry; subjectingthe part thus formed to a single heat treatment having only in astress-relieving treatment, which is carried out keeping the formed partat a temperature in the range from 530° C. to 580° C. for a time in therange from 45 to 60 minutes and then leaving the part to cool in air.

In short, the invention is based on the idea of using a high-strengthsteel, in particular a high-strength low-alloy steel (which, as everyoneknows, is a kind of steel not suitable for quenching), and of carryingout, after the forming step, a stress relieving treatment as the onlyheat treatment, instead of a quenching and tempering treatment. Thestress relieving treatment consists, in per-se-known manner, in keepingthe part for a given time (for instance, 45 to 60 minutes) at atemperature lower than the critical interval of the steel (for instance,at a temperature comprised in the range from 530° C. to 580° C.) andthen leaving the part to cool in air. The stress relieving treatmentallows to reduce, if not even to completely eliminate, the residualstresses generated in the material as a result of processing operations(not of heat treatments) carried out in the previous steps of themanufacturing method (tube making, in case of a part obtained from atube blank, and forming), and therefore to obtain parts having highfatigue life characteristics. In this connection, it is to be noted thatin those parts which are intended to be used on vehicle suspensionsystems the fatigue life is one of the most important requirements. Afurther advantage is that the quenching treatment is avoided andtherefore the associated times and costs are saved. Moreover, the stressrelieving treatment requires temperatures and times lower than thoserequired by the tempering treatment (typically between 590° C. and 610°C. for about 2 hours). The savings in terms of cycle-times and costs(both processing costs and apparatus costs) with respect to the use ofsteels for heat treatment and to the carrying out of a quenching andtempering treatment at the end of the forming steps are thereforeevident. Moreover, the quenching and tempering treatment significantlychanges the metallographic structure of the material and thereforerequires special quality controls on the finished part to check thetreatment has been correctly carried out. The method according to theinvention, on the contrary, does not require these special controls,since it does not provide for subjecting the already formed part to aquenching and tempering treatment, but only to a stress relievingtreatment.

For the purposes of the present invention, the term “forming” is used todesignate any plastic forming process carried out on the sheet metal,such as in particular a tube making process, a shaping process, acold-stamping process or a hydro-forming process. Irrespective of thespecific process (or of the specific processes, as several formingprocesses of different kind could be carried out one after the other)used to form the part starting from a sheet of high-strength steel,according to the invention a stress relieving treatment is carried outas the only heat treatment after the part has been formed, so as toreduce as much as possible, if not even to completely eliminate, theresidual stress state generated in the material as a result of theforming.

Further features and advantages of the method according to the inventionwill be more evident from the following detailed description of apreferred mode for carrying out the method.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The manufacturing method according to the invention provides first ofall for obtaining a part, such as for instance a vehicle suspensionpart, by means of one or more forming operations starting from a sheetof high-strength steel, in particular of high-strength low-alloy steel,such as for instance S600MC steel. The part may be for instance across-member for a twist-beam axle suspension, a longitudinal ortransverse arm for an independent suspension or for an interconnectedsuspension, a frame for a front suspension, etc. In case of a tubular orprofiled part, such as for instance a cross-member for a twist-beam axlesuspension, the method will include first a tube making process (or,more generally, a shaping process) such as the one described in theintroductory part of the description of the present application, so asto obtain a tube blank which will be then subjected to the outrightforming process. The forming process may be performed by cold-stampingand/or by hydro-forming. During the forming process, other operations onthe part, such as for instance bead welding operations, may obviously bealso carried out. Also making welding beads generates residual stressesin the material.

After the forming process (and the bead welding, if any), the part thusobtained is subjected to a stress relieving heat treatment in order toreduce to the minimum the residual stress state generated in thematerial as a result of the plastic deformation undergone during theforming process, and also as a result of the tube making or shapingprocess or as a result of the bead welding. The stress relievingtreatment is for instance carried out keeping the already formed part ata temperature comprised for instance in the range from 530° C. to 580°C. for a time comprised for instance in the range from 45 to 60 minutesand then leaving the part to cool in air. The above-indicatedtemperature and time ranges for the stress relieving treatment are to beconsidered as applicable to high-strength low-alloy steels, such as forinstance S550MC and S700MC steels, and might therefore change in case ofuse of high-strength steels of different kind.

The Applicant has experimentally noticed that the fatigue life of formedtubes made of S700MC steel is increased by about ten times if the tubeis subjected to a stress relieving heat treatment after being stamped.Similar fatigue tests have been carried out by the Applicant on formedtubes made of S550MC steel and have given as a result a fatigue lifeincreased by at least two/three times (the tests have been stopped fortime reasons and no breakages have been detected) with respect to theone of similar tubes not subjected to a stress relieving treatment.

In the light of the preceding description, it is evident that the ideaof subjecting a formed part of high-strength steel sheet only to astress relieving heat treatment after the forming process allows toobtain parts with high mechanical properties, in particular high fatiguelife, in less time and at lower costs than those required in case thepart is subjected to a quenching and tempering treatment after theforming process. The method according to the invention is thereforeparticularly suitable for being used for the manufacturing of parts forvehicle suspension systems, as these parts must meet very strictrequirements in terms of fatigue life and must be produced with thelowest times and costs possible.

The present invention has been described in an illustrative manner. Itis to be understood that the terminology which has been used is intendedto be in the nature of words of description rather than of limitation.Many modifications and variations of the present invention are possiblein light of the above teachings. Therefore, within the scope of theappended claims, the present invention may be practiced other than asspecifically described.

1. Method for manufacturing sheet metal parts of high-strength steelwhich are subject in use to fatigue, the method comprising in the orderthe steps of: a) carrying out one or more forming operations on a sheetof high-strength steel so as to provide the part with the desiredgeometry; and b) subjecting the part thus formed to a single heattreatment having only a stress relieving treatment, which is carried outkeeping the formed part at a temperature in the range from 530° C. to580° C. for a time in the range from 45 to 60 minutes and then leavingthe part to cool in air.
 2. Method according to claim 1, wherein theforming step a) is carried out using a sheet of high-strength low-alloysteel.
 3. Method according to claim 1, wherein the forming of the partcarried out at step a) comprises at least one of the followingoperations: tube making, shaping, cold-forming and hydro-forming. 4.Method according to claim 1, further comprising the step of carrying outat least one bead welding on the part, wherein such a welding step iscarried out before the stress relieving step b).